linux-kernel - Re: [Question] smp_wmb() in prepare

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Message-ID: <20181122123655.GD28270@redhat.com>
Date:   Thu, 22 Nov 2018 13:36:56 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [Question] smp_wmb() in prepare_uprobe()

Hi,

On 11/21, Andrea Parri wrote:
>
> The comment for the smp_wmb() in prepare_uprobe() says:
>
>   "pairs with rmb() in find_active_uprobe()"

it seems that this comment was wrong from the very beginning,


> but I see no (smp_)rmb() in find_active_uprobe(); I see the smp_rmb() in
> handle_swbp(): is this the intended pairing barrier?

Yes, and the comment near this rmb() says "pairs with wmb() in install_breakpoint()",
today this is not right too.

> Which memory accesses do you want to "order" with this pairing?

See 142b18ddc81439acda4bc4231b291e99fe67d507 ("uprobes: Fix handle_swbp()
vs unregister() + register() race") and the comment above this rmb().

Oleg.